Work Package 5 - Groundbased observations in VLF/LF networks


Objectives

The main objectives of Work Package 5 were:

  1. To improve ground-based VLF methods using multi-station observations for the reliable selection of local perturbations in the ionosphere connected with seismic events and differentiate them from global disturbances induced by atmosphere circulation, solar activity, magnetic storms and substorms. Detection of earthquake preparation zone.
  2. The ground-based support of satellite observations for identification of the wave-plasma anomalies recorded above Europe and the Far East. This objective is connected with objective 2 of WP1.
  3. The correlation of spatial-temporal scales of earthquakes related to VLF/LF phenomena with the geological structure and peculiarities of the Earth’s crust.
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Description

For a coordinated satellite and ground-based search for seismic activity, special attention in the project will be paid to the reception of VLF/LF transmitter signals which provide valuable information about plasma perturbations near the upper atmosphere-lower ionosphere boundary. Over the previous nine years, VLF/LF receivers have been successfully operated at seven Japanese stations and at Petropavlovsk-Kamchatsky in Russia. The regular monitoring of the Fat East network has confirmed a definite connection between anomalies in night time LF signal parameters and earthquakes with M ≥ 5.5.

In Europe, research using VLF signals associated with earthquakes started in 2002 in Bari (Italy). The development of the European network began at the end of 2007 with the installation of a VLF receiver in Graz (Austria). In 2008 the European network was significantly enlarged owing to installation of new receivers in Greece, Romania, Turkey, and Central Italy. At the beginning of 2009 receiver stations were installed in Moscow (Institute of the Physics of the Earth) and in Sakhalin (Institute of Marine Geology and Geophysics). These installations have considerably increased capability of the European and Far East networks both in coverage area and in reliability of observations.

Important results in the application of the sub-ionospheric VLF/LF signal propagation method for detecting seismo-ionosperic perturbations have been obtained recently in Japan (Hayakawa et al., 2006; Maekawa et al., 2006; Muto et al., 2009), Italy (Biagi et al., 2004; 2007; 2008) and Russia (Rozhnoi et al., 2004; 2006a; 2006b; 2007a).

The most significant result that has been obtained from observations using the three European VLF/LF stations -Moscow, Bari and Graz concern the earthquake that occurred in L’Aquila (Italy) on April 6th, 2009 (Rozhnoi et al., 2009). Strong nighttime anomalies for long propagation paths together with a shift of evening terminator for short paths were observed for 5-6 days before the earthquake. Direction finding research has showed excellent coincidence with the real position of the earthquake epicenter. This was the first such result.

The recent development of observation systems in Europe and the Far East can provide useful information on the properties and position of the perturbation region in connection with seismic activity. The use of a network of observation makes it possible to separate the local VLF/LF perturbations connected with earthquakes, volcanic eruptions, and tsunami from large-scale or global anomalies related to atmospheric circulation, magnetic storms and substorms, and solar flares or energetic particle precipitation into the atmosphere. By utilising multi-station observations it is possible to determine the area of the earthquake precursor.

Multi-station VLF/LF observations provide a new method to study the effects of ionospheric plasma related to seismic activity. This project will allow us to develop the methodology of analysis of multi-station data and advance the development of the methods for the discrimination of pre-earthquake disturbances in the ionosphere.

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Highlights

Task 5.3 - Performance of regular observations in Moscow and Yuzhno-Sakhalinsk VLF/LF stations

This task also includes model computation for groundbased data. The ground and satellite data are processed by a method based on the difference between the real signal in nighttime and the model one. The model for the ground observation is the monthly averaged signal of amplitude and phase calculated for the quiet days of every month.

A new UltraMSK station has been installed in Kunashir Island, increasing the VLF/LF coverage of the Far East region. This complements the receivers already installed at Sakhalin, Petropavlovsk - Kamchatskiy, and the Southern Kuriles, closer to Sakhalin than to Kamchatka. The new antenna and receiver block purchase were supported by a grant from the Russian Basic Research Foundation. But the results of VLF/LF measurements at Kunashir Island are of great interested for the SEMEP project. These results are submitted to the complex database on characteristics of recorded VLF/LF signals (starting from September, 2011).

Task 5.4 - Processing and analysis of experimental data

The main aim of Task 5.4 is discrimination between local ionosphere perturbations caused by earthquakes, tsunami and volcanoes and large-scale or global disturbances caused by magnetic activity and atmosphere circulation. With this purpose were performed the following analysis: correlation analysis and investigations of tsunami effects in VLF signal.

Correlation analysis
A method of estimating the sensitivity of VLF/LF signals to seismic processes using a neural network approach based on a three-layer perceptron has been developed. This type of network employs the back propagation technique and is referred to as a supervised network. The development of such a network involves two main stages to solve the problem, namely the training of the network and recognition (the prediction itself). In order to train a neural network, we first create a so-called “training set”. The “teacher” specifies the correspondence between chosen input and output data. In our case a representative database has been collected that includes both the LF data received during three-year monitoring (2005-2007) at the station in Petropavlovsk-Kamchatski and the seismicity parameters of the Kuril-Kamchatka and Japanese regions. For recognition of earthquakes 12 time intervals in 2003, 2005, 2006 and 2007 were chosen. Each interval consisted of between 6 and 8 days of data that included the day of seismic events of magnitude M≥5.5. For nine of the twelve time intervals the neural network has recognized successfully changes in LF signal indicating the earthquake of magnitude M ≥ 5.5 a few days before the earthquake. An example where the neural network has detected changes in LF signal indicating the earthquake of M≥5.5 a few days before the earthquake is shown in Figure 5.1. On the upper bar magnitudes of the earthquakes which occurred in the period of analysis are shown. Each column is marked with the following parameters: M – magnitude, H – depth, D – distance from the epicenter to the receiver, R – the ratio of the radius of the zone display precursors to the distance from the epicenter of the earthquake to the axis of the line “transmitter – receiver”. The dashed line represents the threshold at which the M≥5.5. On the bottom bar chart the results of prediction are represented. These results are formed as the output (single neuron) of the previously trained neural network.

Changes in wave power spectra

Figure 5.1: The neural network prediction tests for July 16-23, 2003..

Tsunami effects in VLF signal

The network of VLF receivers sited in the Far East has, for the first time, been used to observe the response of the lower ionosphere to tsunamis resulting from the Simushir (November 15, 2006 Kuril region) and the Tohoku (March 11, 2011 Japan region) earthquakes. Figure 5.2 shows the amplitude and phase measurements of VLF/LF signals on March 11, 2011 from the transmitters - NWC, JJY, JJI, and NPM (the last transmitter is situated in Hawaii) recorded in Petropavlovsk-Kamchatsky and Yuzhno-Sakhalinsk. From results of the Tsunami Travel Time software the tsunami propagates approximately along the Hawaii - Yuzhno-Sakhalinsk path. It is clearly seen that the signals received at both stations (PTK and YSH) are very similar except for those propagating along the NPM-PTK and NPM-YSH paths which show large differences in comparison to the other transmitters. For this particular pair of propagation paths the signal recorded in Petropavlovsk-Kamchatsky travels along an undisturbed path whereas that measured at Yuzhno-Sakhalinsk clearly shows an anomalous decrease in amplitude of about 10 dB together with an increase in phase of up to 50 degrees.

Changes in wave power spectra

Figure 5.2: Amplitude (left) and phase (right) of the signals from four transmitters recorded in Petropavlovsk-Kamchatsky (pink line) and Yuzhno-Sakhalinsk (blue line) on March 11, 2011.

Figure 5.3 shows the waveforms for the phase and amplitude of the nighttime data recorded along the NPM-YSH propagation path. The wavelet spectrograms of the data reveal the frequency of the maximum spectral amplitude in the range of periods of 8-30 min that corresponds to the internal gravity wave periods. These periods are in compliance with the periods observed in data recorded by the DART sensor buoys. A qualitative interpretation of the observed effects is suggested in terms of the interaction of internal gravity waves with lower ionosphere.

Changes in wave power spectra

Figure 5.3: Top panels show the phase (left) and the amplitude (right) of the signal from the NPM (21.4 kHz) transmitter recorded on March 11, 2011 in Yuzhno-Sakhalinsk. Dotted lines are the averaged signals. The middle panels show the signals filtered in the range 0.5-15 mHz. The bottom panels show the wavelet spectra of the filtered signals.

Task 5.5 - Seismological investigation of the Far East region (Sakhalin and the Kirule Islands)

The objective of Task 5.5 was to analyse and characterise the occurrence of seismic activity in the region of South Sakhalin and the Kurile Islands to draw up a new map showing an assessment of the occurrence of a large earthquake.

Seismic process is usually considered as an example of occurrence of the regime of self-organizing criticality (SOC). A model of seismic regime as an assemblage of randomly developing episodes of avalanche-like relaxation, occurring at a set of metastable sub-systems can be the alternative of such consideration. This SEM model is defined by two parameters characterizing the scaling hierarchical structure of the geophysical medium and the degree of metastability of subsystems of this medium. In the assemblage these two parameters define a model b-value.

Thus, in terms of the SEM model the b-value is defined by two parameters, one of them (r) characterizes scaling properties of the medium, whereas the second (p) answers a probability of a continuation of avalanche-like relaxation of metastable sub-systems. Thus it characterizes the degree of metastability of the medium. These two parameters represent a scaling parameter r and a metastability parameter p.

An advantage of such approach consists in a clear physical sense of parameters of the model. The application of the model for parameterization of the seismic regime of the south part of Sakhalin Island is considered. The estimation was carried out using the detailed catalogue obtained from the networks «Datamark» and «DAT». In the version presented below we have examined earthquakes with M ≥ 2.5, 1789 events altogether. The models of space changeability of the scaling parameter are constructed (Figure 5.4).

Changes in wave power spectra

Figure 5.4: Scheme of changes of the values R(φ,λ), spatial component of the SEM model.

The anomalous increase of the parameter of metastability was found in connection with the Gornozavodsk and Nevelsk earthquakes (Figure 5.5). At the present time high values of this parameter occur in the area of the Poyasok Isthmus. This finding is examined in comparison with other indications of an increase in probability of occurrence of a strong earthquake in the South Sakhalin region.

Changes in wave power spectra

Figure 5.5: Temporary component of the SEM model - sequence of values of parameter of metastability p in spatial-temporary surrounding of earthquakes of the catalogue 2. The groups of the events with higher p values (p>0.55), occurred before and after the Nevelsk earthquake, are marked out by the rectangles.

Now the question is, where on Sakhalin we can predict the most likely place for another powerful earthquake to happen, similar to the Nevelsk one? Can we pinpoint the place of the next strong earthquake? In our opinion, the main thread for the nearest few years lies in the seismic zone situated to the South of the Poyasok Isthmus of land (in the vicinity of latitude 48° N). Here we have noticed a distinct region of a seismic quiescence (Figure 5.6).

Changes in wave power spectra

Figure 5.6: Map of earthquake epicenters of the Southern Sakhalin for period 2009-2011. The seismic gap of the second kind is outlined with the dash line.

This study has characterised the spatial distribution of the seismicity in the Sakhalin-Kurile area. New General Seismic Zoning (GSZ-2012) maps have been developed this year by researchers from IMGG FEB RAS together with scientific groups from other Russian Institutes. The maps give a more precise assessment of seismic hazards for Sakhalin and Kurile Islands territory in terms of MSK-64 scale intensities and peak ground acceleration (Figure 5.7).

Changes in wave power spectra

Figure 5.7: New general seismic zoning of the Sakhalin and Kurile islands territory (GSZ-2012 C). Colours correspond to probability of exceeding of the calculated intensity in any point of the zone during 50 years (more than 1 %; the average period of earthquake recurrence being T = 5000 years).

A number of well-known and original methods has been used for expert evaluation of the seismic situation in Sakhalin - Kurile region, including the recognition of the final stage of a strong earthquake preparation. The methods of earthquake prediction/forecasting are based on the following properties of seismic regime: intensity of earthquake flow, the nonlinearity of seismic process, seismic gaps, quiescence and pauses, anomalies of b-value, latent periodicities, end others. In spite of that the temporal regime of the seismicity remains vague, several intermediate-term predictions of the destructive earthquakes at Far East territory were successful (by works of IMGG seismologists). The areas of most probable future strong earthquakes have been indicated as well (Figure 5.6).

Seemingly, thread for the nearest few years lies in the seismic zone situated to the South of the Poyasok Isthmus of land. Figure 5.6 demonstrates that this is a distinct region of a seismic quiescence (seismic gap of the second kind). The modified map of General Seismic Zoning (actually, long-term prognosis map) and the approach to mid - term earthquakes predictions in Sakhalin - Kurile region provide a necessary background for analysis of VLF anomalies in relevance to strong earthquake (electromagnetic precursors).

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